Charged particle based sources, such as synchrotron radiation facilities and free electron lasers (FELs), typically require the use of a periodic magnetic field to control the path of the charged particle beam for photon emission. Generally, the magnetic field is supplied by two opposing arrays of magnets that are placed around the beam path. The magnets are separated by a gap in which the characteristics of the magnetic field are carefully controlled. Certain characteristics of the magnetic field require that the gap between the structures be changed so that there may be a difference in the gap dimension from one end of the structure pair to the other. It is important that this tapering not induce any additional stresses or constraints in the support structure of the magnet arrays or any mechanism for controlling the movement of the magnet arrays. It is also important that the tapering does not induce motion of the magnet structures in any more than one direction. Generally, the motion being described to change the gap is provided by conventional stepper or servo motors that drive lifting screws attached to the magnetic structures.
The ability of a magnetic structure motion control system to tolerate extreme gap taper without damage to the structure is of paramount importance. This is especially so because the provision of a physical taper limiting mechanism within the motion control system is generally considered to be neither reliable nor cost effective.
It is also important to be able to adjust the initial relative angular orientations of the two magnetic structures and then to maintain that angular relationship during any subsequent motion of the two structures.